Previous Versions can be found here.
Knife is a Determination Spell Card, added in Beta 6.6.
Although it is very cheap and can kill any enemy monster instantly, the player will lose HP equal to the Cost of the killed monster as result.
Best saved for rather cheap monsters with high ATK or HP, such as Igloo, Ice Wolf or Napstablook. You can also use it on monsters with a cost that bring you to 1 HP after use, able to fully utilize Last Dream.